OCF Dipole Help

[Jamey Finchum]

I’ve been working on putting up an OCF Dipole on a fiberglass mast in my back yard. I finally have the mast and antenna up, but am having trouble with the SWR being off. It shows the antenna resonant at frequencies that are not available (ie. 1:1.2 at 14.700 MHz).

The instructions for the antenna say to hang the balun at ~30 ft or higher and the ends of the antenna should be 6-10′ off the ground. Of course, when I see “higher” I think “better”, so the balun is at about 35′ and the ends are about 8′ off the ground.  The instructions say the angle of the legs should be between 120-180 degrees. This is where I think my problem lies as I think the angle mine is at is slightly less than 120 degrees.

Knowing this is most likely the problem am I better off lowering the balun a few feet or trying to spread the ends out more (I’m not actually sure I can spread them out much further than they are). Will I be hurting the performance of the antenna by lowering it a few feet? It would still be above 30′.

A bit of bright news is that I also put up a 2m/70 cm antenna up on the mast and am hitting repeaters down near the Rhode Island border!

[Fred Kemmerer]

Hi Jamey,

How does the SWR look on the other bands and in the useful part of 20m (14.000 – 14.350 MHz)? Are the wires near any metal like rain spoutings?

[Jamey Finchum]

Fred,

At 14.200 MHz the SWR is 1:2.1.  Here are some other readings I have:

3.500 MHz (1:2.5)

7.200 MHz (1:1.9)

9.400 MHz (1:1.6)

12.900 MHz (1:1.4)

14.200 MHz (1:2.1)

14.800 MHz (1:1.3)

28.300 MHz (1:1.5)

28.700 MHz (1:1.3)

I do have aluminum siding, but it didn’t seem to bother it before and it’s not really any closer.  I can’t think of anything else that might interfere with it.

 

Thank you,

 

[Fred Kemmerer]

Thanks Jamey. Its possible that your antenna has reasonable SWR values within the Ham bands even though the lowest SWR readings are not in the center of the bands.

Can you also post the SWR readings at a few frequencies within each of the bands that the antenna is supposed to work on:

80m @ 3.5, 3.8 and 4.0 MHz
40m @ 7.0, 7.15 and 7.3 MHz
30m @ 10.130 MHz
20m @ 14.0 14.175 and 14.35 MHz
17m @ 18.118 MHz
12m @ 24.940 MHz
10m @ 28.0, 28.5, 29.0 and 29.5 MHz
6m @ 50.0, 50.3 and 50.7 MHz

I know that this is a lot of readings so the ones to do/post first would be those for 20m and 40m. I think that you had this same antenna up before? Is that right? If so, do you know if you are seeing significantly different SWR values than before? If so, can you describe how the new mounting location of the antenna changed with respect to:

1. Feed point height and proximity to your house
2. End of wires height and proximity to your house
3. Overall length of the coax feedline change

I see from your original post that the center of your new mounting is higher which is good. Before you try lowering the balun, is there any way that you could get the ends up higher? Maybe with some short poles to bring the ends up to about 15′? This would be the way to go if you can get it to work out.

If you would like to talk about this “live”, give me a call. I should be around or available on my mobile for most of the day.

[Jamey Finchum]

Hi Fred,

I raised one end of the antenna’s legs and now have better readings within the ham bands – although would still like to do better.  The balun is up about 35′ and the feed line run straight down the mast and along the ground to the grounding rod (about 15′ away).  The mast is about 2 1/2′ feet from the house (it does have aluminum siding – although I didn’t think aluminum would bother it, but not sure). The longer end leg is about 15′ off the ground now and the short end leg is probably about 18′ or so off the ground. The dips in the SWR readings are much closer to where they should be now.

Here’s what I have:

28.000 MHz (1:1.8)

28.300 MHz (1:1.7)

29.000 MHz (1:1.9)

29.300 MHz (1:1.15)

29.600 MHz (1:1.47)

14.000 MHz (1:1.7)

14.200 MHz (1:1.43)

14.300 MHz (1:1.32)

7.000 MHz (1:1.9)

7.150 MHz (1:1.8)

7.250 MHz (1:1.7)

7.300 MHz (1:1.7)

My antenna analyzer does not go up to 6 meters so I’m not sure of the readings for 6m. This is the 4 band OCF dipole so should only be resonant in 40m, 20m, 10m and 6m.

What do you think? I may try to get the ends just a little higher and see if that lowers the readings a little, but it should be good with a tuner.

Another note, AO-91 made a very low pass a few minutes ago and I could hear it just on the antenna on the mast!

 

Thank you,

[Jamey Finchum]

Hi Fred,

I was able to raise one of the antenna legs up a few feet so both are now 12-15′ off the ground. The balun is up about 35′ and above the roof of the house. The feed line runs down the mast, which is about 2 1/2′ feet away from the house. The house has aluminum siding, but I didn’t think aluminum would affect it… not sure if I am right or not. The feed line then runs about 18′ along the edge of the house on the ground to the ground rod.

I took more readings after raising the longer leg of the antenna up a few feet and got some better results. I am going to try to get the other end up a little higher as well (probably on Friday). Here’s the readings I took today:

28.000 MHz (1:1.8)

28.300 MHz (1:1.7)

29.000 Mhz (1:1.19)

29.300 Mhz (1:1.15)

29.600 Mhz (1:1.47)

14.000 MHz (1:1.7)

14.200 MHz (1:1.43)

14.300 MHz (1:1.32)

7.000 MHz (1:1.9)

7.150  MHz (1:1.8)

7.250 MHz (1:1.7)

7.300 MHz (1:1.7)

My antenna analyzer does not do 6 meters so I’m not sure how it is on that band. The four bands that the antenna cover are 6m, 10m, 20m and 40m.  It looks like raising one leg of the antenna has helped to put the SWR into a usable range in those bands. I’ll see if it lowers it some more by raising the shorter leg up a bit – although this leg is already as high if not higher than the other leg.

On another note for the 2m/70cm antenna. There was a very low pass of AO-91 this afternoon and it picked it up! I’m very impressed with this antenna.

Thank you,

[Fred Kemmerer]

Your readings don’t look too bad and you can certainly use your antenna safely on the bands that its made for. Given you have good quality feedline (LMR-400 equiv.) you should be able to use your autotuner when the antenna’s SWR is above 1.5:1. Given then raising the ends of the antenna helped you SWR, I would suggest you try raising the ends some more if you can and see if that helps.

Glad to hear that you heard AO-91! That is very cool! You might try listening for the other FM sats (AO-85 and SO-50). Do you hear these only when they are close to the horizon? If so, how long does is the period when you can hear them? If its in the range or 3 minutes or more, you might try setting up your radio and making a satellite contact. You IC-7100 should be able to do this if you program a set of memories to handle the doppler shifts associated with these satellites.

Also, would you like to borrow one of our analyzers that does 6m so you can check that band out as well?

Have fun with your new antennas!

[Nashua Area Radio Society]

[attachment file=28797]

Invest in a quality tuner. They are low loss and can match just about anything. I have loaded and made QSOs on a coat hangar.  These days there are automatic tuners so they will instantly pick the proper values transparently whenever you switch between the 11 bands  (6m through 160m).

My two MFJ-94E tuners do a spectacular job matching my verticals on all bands. Since everything beyond the tuner is considered “Antenna” make sure you take steps to reduce any feedline radiation.

LAYNE AE1N

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[Jamey Finchum]

Thanks, Layne! I have the LDG AT-100PROII Auto tuner. I set the radio up today and have been good signal reports on all the bands – haven’t tried 6m yet.  I’m hoping to work some DX this weekend in the WWCQ CW contest!

Thank you and 73,

[Fred Kemmerer]

Have fun Jamey and good DX with your new antenna setup. Anxious to hear how you sound on the repeater net on Sunday.

73,

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